Wave guide attenuator



y 1956 K. MILNE ErAL WAVE GUIDE ATTENUATOR Filed Dec. 10, 1951 a r Y S a b e i W a m O n a United States Patent-* i WAVE GUIDE ATTENUATOR Kenneth Milne, Rugby, and Harry B. Taylor, Ferry Hill,

England, assignors to General Electric Company, a corporation of New York Application December'10, 1951, Serial No. 260,842

Claims priority, application Great Britain December 21, 195 0 2 Claims. (Cl. 333-81) Our invention relates to the art including wave-energy attenuators, and more particularly to variable attenuators for wave guides.

Attenuation of electromagnetic waves in wave guides has heretofore been eifected by the use of attenuators in the form of a dielectric strip having a thin conducting layer deposited on one surface and adapted to be moved in a guide to positions of different field intensity, whereby diiferent amounts of power are absorbed in the conducting layer. Such attenuators have usually been sensitive to atmospheric conditions and thereby essentially non-precision, or they have been fragile and expensive to replace. There is, in addition, a low maximum limit to the power-that can be absorbed in the conducting layer without it overheating and/or burning out.

It'can be shown, however, that theenergy in a wave may be divided by passing it through a circular section wave guide having two mutually perpendicular rectangular stub guides, so called, the wide transverse dimensionsofwhich are parallel'to the axis "of the circular guide, the stub guidestbeing spaced apart-along that axis to reduce mutual coupling. The circular guide is provided with a shortcircuit a quarterwavelength from the stubs; If the circular guide is axially rotatable with respectto the wave, the totalenergy of the wave is divided between the stub guides in the ratio of sin A to cos A, where A is the-angle of rotation from the position where one stub guide receives all the energy. Thus, it will be seen, a desired portion of the energy of the wave maybe delivered to one of the stub guides.

A principal object of our invention is, therefore, to provide novel high-precision wave-guide attenuator apparatusthat are capable of withstanding rough handling and which, at the same time, can absorb large amounts of power without destruction or deterioration. Another object'is to provide a variable wave-guide attenuator having no moving parts other than the body of'th'e attenuator.

According to one'feature of our'invention, an attenuator for electromagnetic waves comp'rises an axially rotatable, hollow, cylindrical conductor adapted to be traversed by the waves, and providedwith a rectangular stub guide, the axis of which is'perpendicular to the axis of thecylindrical conductor and the wide-transversedimansions of which are'parallel to said axis.

The amount of energy transmitted-through the cylinder is inversely proportional to, cr me attenuation is directly proportional to the amount of energy'fed to the stub guide. Thus, the attenuation is zero when the axis of the stub guide'is parallel to 'the'direction of polarization of thewaves for there'can then-beno transverse electric field. The-attenuation is complete when the" axis is rotated to' be perpendicularwith the directionof polarization of the wave.

' Theattenuator,"accordingito our'invention, may be coupledto a coaxial rectangular guide having its long side perpendicular with the axis of the stub. r Such-an arrangement is. referredfto: in this specification as a stub 2,756,396 Patented July 24, 1956 ice unit, and an eflicient attenuator system, accordingly, comprises a serial arrangement of'such stub units, as will appear.

The loss of energy to the stub guide is proportional'to sin A, and if the wave is passed through two such attenuators, in series, the energy remaining is proportional to cos A. Between values of cos A of about 0.1 and 6.9, the variation of cos A with the angle A is substantially linear.

It is, therefore, another object of our invention to provide an attenuator having a predetermined law of attenuation and particularly a substantially linear law. According to a further feature of the invention an attenuator for electromagnetic waves comprises first, second and third lengths of hollow, cylindrical conductor adapted to be traversed by the waves in succession, the second length being axially rotatable relative to the first and third lengths, the second and third lengths each being in the form of a stub unit, as hereinbefore defined.

As the second length is rotated relative to the first length there is a loss of energy to the stub guide proportional to sin A, as above described. However, since the second length is a stub' unit, the long side of the coupled effective rectangular guide also rotates so that the plane of polarization of the waves entering the third length are rotated relative to the third length and hence relative to the stub guide connected thereto. The effect is as if the second length were stationary and the third length rotates relative thereto so that the result is the same as for rotation between the first and second lengths, namely loss of energy in the stub guide proportional to sin A. Thus, the total energy remaining in the guide will be proportional to cos A. As stated above, this loss "law is substantially linear over a wide range of values of cos A.

More specifically, therefore, an object of the invention is to provide an attenuator for a wave-guide transmission system having an input terminal adapted for transmission of ultrahigh-frequency waves characterized by a predetermined direction of polarization, said attenuator comprising a series connection of two stub units, each said unit including a section of circular wave guide having a stub-connected section of rectangular wave guide and a series-connected section of rectangular wave guide, said stub units being rotatable relative to each other, thereby to effect attenuaion of the energy propagated therethrough according to a linear function of'the angle between the axis of the stub-connected section of said rotatable unit and the polarization of the waves at said input terminal.

Where required, a cam or suitably shaped elliptic-gear mechanism may be used to rotate the rotatable unit, whereby the relation between movement and attenuation may be rendered linear over a range wider than that noted above.

Other objects will become apparent and the invention better understood from a consideration of the following description taken in conjunction with the accompanying drawing in which:

Fig. l is an elevational view, partly broken awayy'of an attenuator according to one aspect of the invention, and Fig. 2 is a similar view of a modification.

Referring to Fig. 1, a microwave transmission system is disclosed in which electromagnetic waves originating in a source of UHF wave energy are fed throughxa rectangular wave guide 2, 2' to a utilization device. In order to control the portionof the total energy that reaches the utilization device, an attenuator is coupled into the guide 2, 2.

The attenuator consists of three lengths 3, 4, and 5, of hollow cylindrical guide, the first length 3 being coupled, as by a tapered transformer section .6, to. thatpart 3. 2 of the guide 2, 2' nearer the source, which part we have. termed the entrance guide or input terminal, and the third length 5 is coupled by a similar transformer 6 to that'par't 2 of the guide 2, 2 nearer the utilization device, the so-called exit guide or output terminal.

Thesecond length 4 is electrically coupled between the lengths 3 and 5, althougfh mechanically isolated therefrom,- the length 4 being adapted to be axially rotated relative the lengths 3 and 5. Thus, rotating joints 7 ofany'suitable conventional type are provided, which "are'desirably formed with conventional quarter-wavelength chokes (not shown) between adjacent lengths 3,

"4 and 4, '5 to minimize loss of energy as by leakage or radiation through the joints 7. Rotative movement of the length 4 can be effected by any suitable means, here shown in the form'of gears 20, 21 fixed respectively to the length 4 and a shaft 22. Rotation of the shaft 22 in bearing supports 23 may be effected by a hand crank 24. I As shown in the drawing, the wide transverse dimension of the rectangular guide 2 is in the plane of the paper. For the present description, it will be assumed that the guide 2 is excited in the dominant transverse electric mode TEo,1. The waves are thus polarized in the direction perpendicular to the plane of the paper. The waves 'in the circular guides 3, 4, 5 are also polarized in this direction and the mode may be the dominant TE1,1.

It will be understood that, if required, polarizing means such as a conductive rod 25 or another suitable polarizer can 'be provided in length 3 adjacent the joints 7 to minimize undesired rotation of the polarization of waves therein due to the presence of the joints 7 and/or other unavoidable irregularities in the length 3. In the absence guide length 4 and communicating therewith through a rectangular slot 26 formed in the wall of the length 4. The axis of the stub guide is perpendicular to and the wide transverse dimension parallel with the axis of the guides 2, 3, 4 and 5. The stub guide 8 is terminated in a matched load 9 for complete absorption of energy launched therein.

The part of the circular guide 4 adjacent the length 5 contains a polarizing strip 10 of suitable conducting material, which is desirably spaced substantially a quarter wavelength from the slot 26 and extends diametrically across the guide ina plane perpendicular to the axis of the stub guide 8. The ends of the strip 10 are desirably tapered to minimize reflections. The effect of the strip 10 is to cause that part of the guide 4 occupied thereby to act as a rectangular wave guide, the wide transverse dimension of which is parallel to the plane of the strip 10 and perpendicular to the axis of the stub guide 8.

It will be noted that the combination of circular guide length 4 and its stub-connected rectangular guide 8 together with the effective rectangular wave guide 10, 4 corresponding to that part of the guide length 4 occupied by the strip 10 is, in effect, a stub unit, as herein defined.

The third lengthS of circular guide is provided with a similar rectangular stub guide 11, the axis of which is perpendicular to the wide transverse dimension of the guide 2. This guide 11 is also terminated in a matched load 12. To minimize undesired rotation of the polarization of the waves in the length 5, the stub guide 11 is desirably positioned as close to the flange joint 7 as is convenient. Being coupled to the rectangular guide 2, the length 5, in combination with rectangular guide 2, is electrically a unit similar to the length 4, and accordingly is a stub unit connected in series with the stub unit provided by length 4.

The operation of the attenuator apparatus of Fig. l

' energy entering the entrance guide 2, so that the total energy entering the attenuator is disposed between the the wide transversefldimension of .the guide 2, and the plane of the strip 10 is parallel thereto, e. g., occupying positions at right angles to those shown in Fig. 1, no transverse component of electric field is set up in the stub 8, and hence no energy is fed to-and absorbed in the load 9. The total energy passes the stub 8, the, strip 10, the length 5, and'through the exit guide 2, in succession, to the utilization device, unafiected bysuch transit since no energy is extracted 'by the stub guide '11, inasmuch as the latter is similarly situated withrespect-tothe direction of the polarization of the waves. 1 l 7 If, now the length 4 is rotated through an angle A (less than the electric field in the guide length 4 has a component, sin A, normal to the wide transverse dimension of the stub guide 8 and thus, since energy is proportional to voltage squared, a portion, sin A, of the energy entering the guide 2 passes into the stub guide 8 to be absorbed in the load 9. The remaining portion, cos A, of the energy proceeds down the length 4.

The effective rectangular guide 10, 4 formed by the strip 10 in conjunction with the circular guide 4, it will be observed, has also been rotated through the angle A, and the electric field component thereat, cos A, is parallel to the wide transverse dimension of the elfective rectangular guide 10, 4, whereby energy proportional to cos A passes through this part of the guide 4. Since, as noted above, cos A is the portion of energyremaining after the extraction by the stub guide 8, the total energy is disposed of and no reflections are produced.

But the guide length 4 has also been rotated through the angle A relative to the length '5 of circular wave guide and the electric field set up in the guide length 5, being parallel with the wide transverse dimension of the equivalent guide 10, 4, is at the angle A- to the axis of the stub guide 11. Hence, the field has a component sin A transverse thereto. In the same manner as before, a portion sin A of the incoming energy is fed to the load 12 and'a portion cos A remains in the guide 5 as the residual energy. Since the component of electric field perpendicular to, the wide transverse dimension of the exit guide 2' is cos A of the total field, a portion cos A of the energy entering length 5 is finally transmitted. This is a portion cos A of the loads 10 and 13 and the receiver 3, there being no reflections.

Since, as has been pointed out, cos A is substantially linear with respect to A over a widerange of values of cos A, a predetermined attenuation is obtained for a given rotation of the unit 4.

Although cos A is substantially linear over a wide range, where it is desired to have more perfect linearity of attenuation with movement, elliptic-gear arrangements, as shown at 27, may be used to drive the length 4. Such gears may be so shaped that uniform motion of the crank 24, although causing variable motion of the length 4, results in uniform, variation in attenuation.

As will be understood from the foregoing description,

attenuation is achieved in a guide wherein the source and the utilization device have no relative movement. If it is not necessary thatthe utilization device be fed by a stationary guide, but instead can be directly connected to a rotating member, or if it is desirably so connected, the attenuator can consist merely of thepart shown on the source. side of the line 13.

Such an attenuator is illustrated in Fig. 2, and, as

shown, the guide 32 coupled to the utilization device is,

coupled to the guide length 34 to be rotatable therewith. The details of construction of the guide length 34 as well as the other elements of the attenuator of Fig. 2 are similar to the corresponding elements described in relation to Fig. 1 and, accordingly, a full description thereof is unnecessary.

The operation of .this embodiment of our invention is generally similar to that described above in connection with the corresponding portion of Fig. 1. Thus, of the polarized wave energy applied from the source and entering the entrance guide 42, energy proportional to sin A is fed to the load 36 of the stub guide 38 and the residue cos A is fed to the utilization device. The orientation of the guide 34 coupled to the utilization device is determined in accordance with the amount of attenuation desired.

It will be apparent that motive means generally similar to that illustrated at 20, 21, and associated elements of Fig. 1, can be employed to provide any suitable rotative movement of the guide length 38.

In the event that a symmetrical attenuator is desired or required for use in reversible systems wherein the impedance presented from either terminal is desirably identical, an additional relatively stationary stub-connected guide can be provided, similar to stub guide 11 (Fig. 1), the additional guide being connected to the length 3 so that the axis of the additional guide is normal to the wide transverse dimension of rectangular guide 2. Also, an additional relatively rotatable stub-connected guide can be provided similar to stub guide 8 and connected to the rotatable length 4 and adjacent the end of the strip opposite the stub guide 8. The latter-mentioned additional stub guide is connected so that the axis thereof is parallel to the axis of the stub guide 8 and both said axes are normal to the strip 10. It will be apparent that the additional stub guides are thus disposed to affect the polarization of waves in a manner similar to that of the corresponding similar stub guides, whereby the operation of the attenuator is virtually unaffected except to make it entirely symmetrical so that equal impedances are presented when viewed from either terminal.

There has thus been described an attenuator unit that provides attenuation of UHF wave energy in accordance with a predetermined law of attenuation and which can be fabricated to withstand rough handling since there are involved no moving parts other than the unit as a whole. Furthermore, our apparatus has the advantage over attenuators previously known, in that it is possible to dispose of as much power as desired in the matched loads so that there is virtually no limit to the powers with which the attenuator can be used.

While we have shown and described specific embodiments of our invention, we do not desire our invention to be limited to the particular form shown and described and we intend, by the appended claims, to cover all modifications within the spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An attenuator for a wave-guide transmission system having first and second stationary rectangular wave-guide sections, said first rectangular section defining the input terminal and said second rectangular section defining the output terminal for said system, said attenuator comprising a first cylindrical wave-guide section rotatably coupled to said first rectangular wave-guide section, a first rectangular stub guide connected to said first cylindrical wave-guide, the axis of said stub guide being perpendicular to the axis of said cylindrical Wave-guide, means in said stub guide for absorbing a component of energy propagated through said cylindrical wave-guide according to a function of the angular displacement of said cylindrical guide relative to said first rectangular section, a second cylindrical wave-guide section coupled between said first cylindrical section and said second rectangular wave-guide section, said first and second cylindrical wave-guide sections being coaxially aligned, a second rectangular stub guide connected to said second cylindrical guide, the axis of said stub section being perpendicular to the axis of said cylindrical guide, said second rectangular stub guide having means for absorbing energy propagated through said second cylindrical guide according to a function of the angular displacement of said second cylindrical section relative to said first cylindrical section.

2. In a wave energy attenuating system, input and output terminals, first and second cylindrical waveguide sections positioned in coaxial alignment, means for rotatably coupling said first cylindrical wave-guide section to said input terminal and to said second cylindrical wave-guide section, means for coupling said second cylindrical wave-guide section to said output terminal, means for rotating said first cylindrical wave-guide section through a predetermined angle A with respect to said input terminal and said second cylindrical wave-guide section, means coupled to said first cylindrical wave-guide section for absorbing energy proportional to sin A and passing energy proportional. to cos A, said last-named means comprising a rectangular stub section having an axis perpendicular to and a wide transverse dimension parallel to the axis of said first cylindrical wave-guide section, a second absorbing means coupled to said sec- 'ond cylindrical wave-guide section for absorbing energy proportional to the sin A and passing energy proportional to the cos A, said second absorbing means comprising a rectangular stub section having an axis perpendicular to and a wide transverse dimension parallel to the axis of said second cylindrical wave-guide section, whereby the total energy remaining at said output terminal is proportional to the cos -A.

References Cited in the file of this patent UNITED STATES PATENTS 2,129,712 Southworth Sept. 13, 1938 2,422,601 Tashjian June 17, 1947 2,425,345 Ring Aug. 12, 1947 2,438,119 Fox Mar. 23, 1948 2,531,194 Bowen Nov. 21, 1950 2,603,709 Bowen July 15, 1952 2,606,248 Dicke Aug. 5, 1952 

